WIP: Add /?action=snapshot URL to HTTP server

[virtuald]

trafficstars

Fixes #62

This works great and provides consistent images if there's already an active stream. However, if there isn't an active stream, if exposure is low then you get strange images at first -- I presume because the settings are taking some amount of time to settle?

Obviously, the current implementation is quite a hack, but it's a start. Any ideas on how to fix it?

[virtuald]

@PeterJohnson I made the changes you suggested. It seems better, but the delay is annoying. However, I'm willing to live with it for now I think.